The MBEC and MCIBF should be co-linear. I would propose plotting them together and looking for the best fitting regression. You could investigate MBEC as a response to changes in MCIBF and other variables such as strain type, biofilm age etc. But make sure you explore the data for co-linearity first. Follow Zuur et al. (2010; DOI: 10.1111/j.2041-210X.2009.00001.x) as a guideline for the statistical exploration

Second part, I would suggest a linear regression to look at the relationship between these two variables and quantify the coefficient of variation (R-squared value).